1. Field of the Invention
This invention relates to compact antennas capable of operating efficiently at a number of separate frequencies that need not be harmonically related and which may be relatively close together.
2. Description of the Prior Art
Many types of multiband antennas have been developed, some making use of arrays of antenna elements and some making use of one of various switching arrangements. U.S. Pat. No. 2,535,298 to Lattin describes a simple multiband dipole antenna to which are connected quarter wave sections that function to cause the antenna to resonate at different predetermined frequencies. The central section of the dipole is caused to resonate at the highest frequency of tha antenna and quarter wave sections, measured at this highest frequency, are connected to the ends of this central section. The next highest frequency is determined by the central section of the doublet and the quarter wave sections plus two additional sections, one on the outside of each of the aforesaid quarter wave sections, and, if the antenna is to be adjusted to function at still another frequency, that is, a third frequency, then two additional quarter wave sections, measured at the aforesaid second frequency, are connected to the ends of the antenna adjusted for the second frequency, so that additional sections may be connected to the ends of the latter quarter wave sections to adjust the antenna for the third frequency.
In the Lattin structure, the more less independent radiating sections are achieved by using parallel quarterwave transmission lines as isolating elements. Each quarter wave section consists of a pair of side-by-side spaced conductors that are shorted together at the antenna ends furthest from the central feed point. Additional radiating elements extend from the point where the conductors are shorted to form a radiating section encompassing the entire antenna structure that resonates at a lower frequency than the central dipole antenna. In the structure described by Lattin, the operating length of the dipole at the lower frequency is two or more times the operating length of the dipole at the higher frequency and the antenna is not suitable for use with frequencies separated by a ratio of less than approximately two to one.
U.S. Pat. No. 2,996,718 to Foley describes a broad band monopole antenna consting of a number of concentric layers of conductive materials each having a different length and each coupled to a receiving or transmitting device. The individual elements, however, are not isolated from each other and operation of the antenna is restricted to less than approximately a 2:1 frequency range.
U.S. Pat. No. 2,648,768 to Woodward describes a wideband dipole antenna in which a desired field pattern is maintained by wire radiating or receiving "open loops" connected to the main dipole elements. Improved results are said to be achieved by arranging the auxiliary conductors to extend outwardly at an angle from the primary dipole elements. The antenna is useful over a predetermined frequency range, but is not effective for handling multiple bands at widely separated frequencies.
U.S. Pat. No. 3,139,620 to Leidy and Cubbage describes a multiband coaxial antenna formed by positioning a pair of stubs on a member, the first a predetermined distance to one side and the second a predetermined distance on the other side of the center of a radiating section. Each stub, which is a quarter wavelength long, includes a shorting washer and at its lower edge is connected through the washer to the member and hence at its upper edge presents a high impedance to a first band of frequencies. The stubs are positioned on the member so that the section between the stubs functions as a dipole for transmission of a first band of frequencies. Another pair of tubular stubs are similarly positioned on either side of the center of another radiating section on the member to provide transmission on a second band of frequencies. In a center fed embodiment, a pair of stubs connected shorted edge to shorted edge are located on each end of the antenna to provide a high impedance over a broad band of frequencies. A whip is positioned on one end and the other end of the antenna is grounded.
The choke arrangement used by Leidy and Cubbage is similar to that described by Lattin with the exception that the chokes are coaxially positioned rather than in series. A common disadvantage of these structures is that the exterior of the choke does not form an effective part of the radiating element at the choke resonant frequency. Therefore, the physical length of the antenna is significantly greater than the operating length, which is a multiple frequency dipole arrangement means a size 50% greater than the effective length of the lowest frequency dipole if its separation between the lowest and the next lowest frequency is 2:1 or greater, and much larger if the frequency separation is less than 2:1.
Other antenna constructions have been proposed with various means of isolation to permit operation of the antennas over a relatively broad band of frequencies or at different separated frequencies. In most instances, either the frequencies must be harmonmically related or the efficiency of the antenna is lowered. Many of the structures, while achieving desirable operating characteristics, are either expensive to construct or present bulky or unwieldy structures that are difficult or expensive to mount.